In communication systems, a network topology, such as a star, tree, or mesh topology, is used. Furthermore, in communication systems, the components of a network, such as computers or hubs, are called nodes.
In a mesh network, in which nodes communicates equally with each other, nodes adjacent to each other communicate. Accordingly, in a mesh network, when compared with a star network or tree network, it is easy to acquire the received electric field indication (RSSI) of each transmission signal from each node. However, in a mesh network, it is difficult to estimate when a node will communicate. Accordingly, nodes need to be in the state such that they can always communicate and thus it is difficult to put the entire network to sleep, which makes it difficult to implement low electrical power consumption.
In contrast, in a star or tree network, a higher level node manages a lower level node. Accordingly, it is easy to synchronize nodes in the network, which in turn makes it easy to implement low electrical power consumption network with a star or tree topology.
However, in a star or tree network, each node only communicates with a node that corresponds to a master station located higher than the subject node when the subject node is used as a slave station or only communicates with a node that corresponds to a slave station located lower than the subject node when the subject node is used as a master station. For example, a slave station located at the end of the star or tree only communicates with a hub that corresponds to a node functioning as a master station. Specifically, communication is not directly performed between nodes that are both functioning as slave stations of a hub. Accordingly, for the slave stations of the hub, it is difficult to obtain RSSI values for slave stations other than the subject hub. Therefore, it is difficult to use, in a star or tree communication system, a positioning algorithm that is a technology used to estimate the distance between slave stations on the basis of the RSSI value between the slave stations and then estimate the location of each slave station by using the estimated distance.
There is a proposed technology for use in conventional mesh network technologies for obtaining the distance between wireless communication terminals from the indications received from communications between wireless communication terminals, for comparing the distance with the result of the positioning performed using GPS, and for correcting the positioning results obtained by the wireless communication terminals (see, for example, Japanese Laid-open Patent Publication No. 2006-343161). Furthermore, there is a conventional technology for obtaining received electric field indications by receiving, at a base station, a response signal from each mobile station in response to a call from a central office and for estimating the location of the mobile stations from the obtained received electric field indications (see, for example, Japanese Laid-open Patent Publication No. 08-172663).
However, as described above, it is difficult to implement low electrical power consumption in a mesh network. Accordingly, for example, if a transmitter used as a slave station is attached to a cow in a field, the battery in the transmitter soon runs down and thus information may not be obtained. Furthermore, in the conventional technology for obtaining received electric field indications by receiving, at a base station, a response signal from each mobile station in response to a call from a central office, because the distance between mobile stations is not obtained, the accuracy with which the location of the mobile stations are specified becomes low.